The concept of c-decay, the slowing of the speed of light, was first proposed by Barry Setterfield in 1981 in an article for the Australian creationist magazine, Ex Nihilo. He selected a number of historical measurements of c starting with the original measurement by Ole Rømer in 1667, and proceeding through a series of more recent experiments, culminating in "modern" measures in the 1960s. These showed a decreasing speed over time, which Setterfield claimed was in fact an exponential decay series that implied an infinite speed in the not distant past1. The claim was later expanded to cover an apparent similar decay of several other physical constants2. Setterfield argues that this resolves the so-called "starlight problem".

As Setterfield's original suggestion in Ex Nihilo notes, "If you propose that the universe and all in it is the product of an act of creation only 6-7000 years ago, many people ask - 'How is it that objects millions of light years away can be seen? Surely such light would take millions of years to reach us." If c is a constant, as is widely accepted, then this implies the universe is billions of years old because we can see objects billions of light years away. However, if the speed was significantly faster in the past, as Setterfield argues, then the light would have traveled most of this distance in a short time. Setterfield proposes this as an alternative to mainstream physical cosmology and, as such, c-decay represents a unique creationist cosmology3.(4)

Theoretical problems abound with the idea that the velocity of light might be changing with time. Einstein's famous theory of general relativity is the most serious challenge. To remind you, the theory is reproduced here: [E = mc2, where E = energy, m = mass, and c = velocity of light]

Although still called a "theory", Einstein's equation has survived the tests of nearly a century of observational confirmation. The equation above shows that any change in the velocity of light results in huge changes in either the amount of energy or mass in the universe. For example, a modest 2-fold decrease in the velocity of light would result in a 4-fold decrease in the amount of matter, or amount of energy in the universe. There is no evidence that any of these kind of changes are occurring in our universe.(5)